Although I usually close mine, I do keep them open on occasion... I dont know why, but sometimes I can just think better or visualize better with them open.
When I close them I sort of go into "REM" mode, and my eyes twitch a lot more. Sometimes it is harder to focus like this, especially if my eyelashes rub against the inside of the blindfold, which can be irritating. With my eyes open, I can just stare straight ahead and sort of forget everything else... sort of like when you stare off into space.
And anyway, I like seeing the wonderful green-teal color of the inside of my blindfold.